The SPOD eigen-spectrum is shown in figure REF . Unlike subsonic and ideally expanded supersonic jets [1]}, the spectrum displays a sharp peak around \(St=0.26\) , associated with the screech tone. This tone is a direct consequence of an absolute instability mechanism induced by the shock-cell periodicity, where flow disturbances grow in both downstream and upstream directions [2]}. Since PFE can only obtain downstream-travelling waves, the flow structure associated with this tone will not be further analysed herein.
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